1. Field of the Invention
The present invention relates to a swimming fin and its method of manufacture.
2. Description of the Prior Art
Swimming fins have to meet several technical and commercial criteria which may be defined in terms of performance, comfort, lightness and esthetics.
State of the art solutions have principally taken into account the problems of performance and comfort.
The performance of a fin is related to the surface area and to the nerviness or stiffness of its blade.
In order to obtain a nervy or stiff blade, it is necessary either to employ a rigid material, such as a composite material based on glass fiber, or else to employ a material of the synthetic polymer resin type which will be stiffened by longitudinal ribs.
However, a rigid blade is generally somewhat incompatible with a comfortable shoe, more particularly for the fins called "snug-fitting" fins where it is necessary to deform the shoe in order to insert the user's foot.
It has therefore been proposed to employ two materials, one a rigid one for the blade and the other a flexible one for the shoe and the state of the art proposes various means capable of solving the problem of good flexible-shoe rigid-blade energy transfer.
Thus, Patent Application EP-A-O,380,333 proposes a fin where the "shoe" is reduced to its simplest expression since it comprises straps crossed over the user's foot.
More generally, the fin is produced by molding the shoe onto the blade. However, it is necessary that the material employed for the shoe to be sufficiently flexible in order to absorb the deformations of the blade in finning action, without giving rise to mechanical tearing.
A first solution, described especially in Patent Applications FR-A-2,173,504 and FR-A-2,213,072, consists in mechanically anchoring the shoe in the blade.
A second solution consists in employing, for the blade and the shoe, chemically compatible materials so as to ensure good cohesion by molding of the shoe,
In this case, an ethylene/vinyl acetate (EVA) copolymer will generally be employed for the blade and a thermoplastic polymer, principally polypropylene-based, "elastomer" for the shoe.
It is also possible to employ, for the blade and for the shoe, polyurethane-based polymers, although it is difficult to obtain a polyurethane sufficiently flexible for the production of "snug-fitting" fins.
Although twin-material fins of the EVA/"elastomer" type are satisfactory in terms of performance and comfort, as in terms of cost price (approximately 4 times less than for fins made of polyurethane), they do not enable the problems of lightness and esthetics to be taken into account.
Lightness becomes a problem to take into consideration when it is desired to produce large-sized blades. In order to maintain good performance, it is then necessary to increase the thickness of the ribs ensuring the nerviness and, possibly, the number of them, thereby leading not only to an increase in the weight, but also an increase in the manufacturing time for the blade.
Finally, the esthetics of the fin become a technical problem when the color or shiny effect or the design which are desired are related to the material employed, it being possible for the latter to be prejudicial to the performance or to the lightness of the fin.
Thus, it is not possible to produce a shiny blade made of EVA, unless it is employed as a mixture with vinyl, thereby leading to a drop in performance requiring a significant increase in thickness of the ribs, and therefore in the weight of the fin and its manufacturing time.